Reducing dependencies for docs

[Dan Smith]

Would it be possible to adjust the build system's handling of docs so that it is not necessary to do a complete compilation before the docs can be generated?

I'm working with specs, and the following command does mostly what I want:
make jdk.jdi-gensrc hotspot-gensrc && make docs-jdk-specs-only

But it would be nice to not have to resort to '-only' targets, when there's no real dependency on 99% of what happens when I use 'make docs-jdk-specs'.

Specific proposals:

- Change docs-jdk-specs so that it only depends on the gensrc phase. (I mean the actual source-generation phase, not the 'gensrc' target. For some reason when I do 'make gensrc', I end up waiting for lots of compilation, including java.base and jdk.internal.vm.compiler.)

- Perhaps gensrc is not the right mechanism for what jdk.jdi and hotspot are trying to do? I don't know the details, other than that each produces a needed html file. The html generation could be made part of the 'docs-jdk-specs' target directly.

- Can we similarly eliminate dependencies from 'make docs'? Right now, the javadoc invocation requires compiled classes to be available, but I don't know if that's necessary. Maybe it will work just as well if it only has sources?